The present invention relates generally to digital demodulators, and more particularly to a digital demodulator comprising A/D converters for multilevel discrimination and digital transversal equalizers for compensating for intersymbol interference.
Analog-to-digital converters employed in a digital demodulator use (n+1) bits or more for discriminating the 2.sup.n levels of a demodulated 2.sup.2n -QAM (quadrature amplitude modulation) signal and representing the discriminated level with the higher n bits which form a main data signal corresponding to the original data signal. The (n+1)th or lower bit of the A/D converter output indicates the amount of deviation of the discriminated level from an optimum level and is used to derive a control signal for use in a closed loop circuit.
Transversal equalizers with variable tap weights are in current use for adaptively compensating for intersymbol interferences resulting from distortion of received digital signals. If transversal equalizers are used to process the output of an analog-to-digital converter, the variable tap weights are controlled by control signals which are derived from logical products of the most significant bit of the higher n-bit and the (n+1)th lower bit of the A/D converter output. One advantage of the transversal equalizers lies in their abilities to compensate for a small deviation of demodulated signal at each sample point.
The error signal is also used in an eye detection clock synchronizer which is capable of generating jitter-free clock pulses. Using the MSB of the main data signal that arises one clock before or after a sample point, the clock synchronizer estimates the polarity of time differentiation of a demodulated baseband signal at the sample point and determines from the estimated polarity and the error bit on which side of an optimum point the sample point is located. A control signal is derived from this determination to establish clock synchronization.